Electrostatic microammeter



Nov. 5, 1957 T. A. RICH ELECTROSTATIC MICROAMMETER Filed March l5, 1954 3 Sheets-Sheet 2 Iii Ihvenizlor: Theodore A. Rich, by

` His Attorney.

Nov. 5, 1957 Filed March 15, 1954 T. A. RICH ELECTROSTATIC MICROAMMETER 3 Sheets-Shea?l 3 log His Attorney- United States Patent C),p

2,812,443 ELECTROSTATIC mCRoAIsn/IETER Theodore A. Rich, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application March 15, 1954, Serial No. 416,270

11 Claims. (Cl. Z50-83.6)

The present invention relates to radiation measuring instruments.

More particularly, the invention relates to an instrument for measuring the intensity of penetrative radiations such as alpha, beta and gamma rays existing in a particular locality.

There are many known radiation measuring instruments available in industry today for measuring the intensity of penetrative radiations such as alpha, beta and gamma rays, and while these instruments are suitable for many purposes, in some respects they have not been entirely satisfactory. More specifically, the response of many of the known instruments of this typeis adversely affected by changes over a period of time, due to ageing and the like, in the value of the energizing potential supplied tothe instruments. Also, many of the instruments are not sufficiently sensitive to low intensity radiations, and are too readily affected by shock and vibration.

It is therefore one object of the present invention tol provide a highly sensitive radiation measuring vinstrument.

Another object of the invention is to provide airadiation sensitive instrument which is substantially independent of changes in the value of the energizing voltage supplied thereto. f

Still another object of the invention is to provide an improved radiation measuring instrument which is dependable in operation, and which is not substantially affected by shock, mechanical vibration and the like. Y

A still further object of the invention `is to provide a radiation measuring instrument wherein extremely large changes in voltage are available for moving an indicating element to indicate relatively small changes in radiation intensity, and wherein the torque required to overcome friction of the indicating element of the instrument is relatively small in comparison to the large changes in voltage available to overcome such torque. In practicing the invention, `a radiation measuring instrument is provided which includes first and second radiation sensitive chambers connected in series electrical circuit relationship with the second chamber having a predetermined amount of radioactive material disposed therein. Movable means are operatively coupled to the first and second radiation sensitive chambers in a manner toy be responsive to .current flow through the chambers for indicating the presence of the penetrativeradiations. In one embodiment of the invention, the radiation sensitive chambers comprise an electrometer having at least two sets of spaced apart fixed plates, and a movable vane disposed in the spacebetween the spaced apart `plates and adapted to movevtherebetween. A radioactive material is disposed in the inner surface of at least one of the sets of spaced apart fixed electrometer plates. In another embodiment of the invention, the firstand second radiation sensitive chambersk comprise `ionization chambers connected in series circuit electrical relation- ICC ship with one of the ionization chambers having radioactive material disposed therein with a conventional quadrant electrometer connected to the ionization chambers for indicating the voltage distribution between the chambers.

Other objects, features and many of the attendant advantages of this invention will be appreciated more readily as the same-becomes better understood by reference to the following detailed description, when considered in connection with the accompanying drawings,

wherein like parts are identified by the same reference character, and wherein:

Fig. 1 is a schematic view of radiation measuring instrument constructed in accordance with the present invention; v

Fig. 2 is a graph of the current and voltage relation existing in a conventional ionization chamber;

Fig. 3a-3d is an idealized graph ofthe current and voltage relation existing in a radiation measuring instrument such as illustrated in Fig. l of the drawings, the actual graph would show the two straight line portions merging without a sharp break;

Fig. 4 is a graph of the torque per volt squared versus deflection of a movable indicating element characteristic of the measuring instrument shown in Fig. 1;

Fig. 5 is a second form of a measuring instrument constructed in accordance with the present invention;

Fig. 6 is a schematic View of still another embodiment of the measuring instrument constructed in accordance with the invention;

Fig. 7 is a cross-sectional view of a radiation measuring instrument designed for practical use in the field and incorporating the features of the present invention; and v l Fig. 8 is a cross-sectional view of the radiation measuring instrument shown in Fig. 7 of the drawings, and taken through plane 8 8 thereof.

The radiation measuring instrument illustrated in Fig. 1 of the drawings includes a first radiation sensitive chamber 11 that preferably comprises an ionization chamb er formed by an outer cylindrical electrode 12 and an inner rod-like electrode 13.` A second radiation sensitive chamber is connected in series electrical circuit relationship with the rst chamber 11 and comprises -a portion of an electrometer having at least two sets 15 and 16 of spaced apart fixed plates. A coating of radioactive material 17 is disposed over the inner surface of the set of fixed plates 16 and a source of energizing potential 18 is connected across or between the two sets of spaced apart fixed plates 15 and 16 the latter of which constitutes the second radiation sensitive chamber means. The set of fixed plates 15 is connected directly to the outer cylindrical electrode 12r of thev first radiation sensitive chamber 11. The inner electrode 13 of the chamber 11 is connected tothe lever arm 19 of a moving element for indicating the presence of penetrative radi-ation which comprises a movable vane 21 disposed in the rspace between the spaced apart plates 15 and 16, and movable therebetween. The movable vane 21 is electrically connected to theinner conductive rod 13, and is subjected to the electrostatic field produced by the potential gradients existing between 'the inner surfaces of the fixed electrometer plates 15 and 16.

The current versus voltage relationship existing in the standard ionization chamber such as that shown at 12 and 13 in Fig. l, is illustrated in Fig. 2 of the drawings wherein it can be seen that as a rough approximation, the current in the ion chamber isrepresented by substantially two straight lines. At lower voltages, the slope of the lines is proportional to the square root of the radiation intensity, and at higher excitation voltages, a sat,

Patented Nov. 5, '?V

I.; uration current is obtained which is proportional to the radiation level. For different radiation'levels, this saturation occurs at diierent values as is illustrated.

In the arrangement shown in Fig. l of the drawings movable vanek 21 and the 'fixed electrometer plates 16 operate as' an ion chamber lso that in ellect, there are two ionization chambers connected in series' circuit re-i lationship. With this arrangement "the distribution of voltage across the two chambers thus formed can be determined -graphically in the manner shown in Fig. 3 of the drawings;

Theelements 21, 19; and 13 are highly insulated and it therefore follows that Ain any stable position the net current llow to` this" system must he zero. 'If the ma'- terial 17 is an' alpha emitter, ionization 'due to 'it' will be very largely concentrated in. the spacebetween plates 1'6. The plate 21 will `receive ion 'current primarily from' that volume 'of airlying' between 21 and 16', and substantially all of this can be considered due to 17 undert the assumption thatthe 'volume'. of '12-A is very much greater than the volumebetween plates' '16. Therefore, there is; apositive ion tlow toV 1`3"frorn the chamber 12 (and a `smaller positive ion llow from 15 "to 21, which is `negligible incomparison to the other) Awhen"radiation is present. There is also 'anegative ion ilow to 21. due to the ion pairs created betweenplates 16 "duetofthe radioactive material 17. The' positive ioncurrent 'isi a function `of 'the external radiation present and the voltage between 12 and 13, and lthe negative io'n current is a function-of the position of 21 and thevoltage between 21 and 16; asset'forth inthe following relations:V

where I+=positive ion current L -Lneg'ative ion current Vi=voltage 15 to 21 V2=voltage 21 to 16 R`=radiation 6=angle of dellection Under these. conditions it can be shown that 9=f(R-), thatis, lthe `angle of deflection is a functionof the radiationlpresent.

InFig. `3. thefelement Y21' is assumedto'be in axed 'angularposition and the heavycurve (L) is aifunctiontonly of V2., The positive. currents for different values of R` `ing a movablevane llwhich-performs the additional function of producing a stableequilibrium position for any radiation level.

The frstof the radiation sensitive devicesencom'passes an ionization chamber 1.1 constituted of a cylindrical outer member 12 and an inner rod-likemember V13. Thision chamber, in conjunctionV with the fixed plates 15 t and` the-movable vane 21 of the electrometer, comprisesfthe first radiation sensitive means.

These two elements produce a flow orpositivefion current upon the impingementof radiation. That i's,plates' 15 andthe cylindrical-chamber 12f are connected to'the positiveterminal of a battery` 18. As a consequenceythe inner rod member..13,-as well as .the movablefvane member 21, are at a potential which is negative..relativ-etc` these aforementioned outer members. Hence,- should radiation impinge onthese chambers andtproduceion pairs; i. e., both positive and negative ions,.the positive` ions will tlow towards' therodfmem'ber 1`3 andthe vane 21 thus causing a positive ion current to flow. Since the chamber 11 is substantially larger than the space between the plates 15, the positive ion current that ilows can be considered as substantially that due to the chamber 11. rThis will become of some significance in discussing, subsequently, the eifect of the position of the vane 21 with respect to the plates- 15.

The plates 16, on the other hand, constitute the source of negative ion current in this system. The plates 16 contain a coating of radioactive material 17 over the inner surface thereof. Since the plates 16 are connected to ground potential, the vane 21' will have a potential with respect thereto which is positive. As a consequence, any ionization occurring in the chamber due to the emission from the radioactivematerial 17 causes the negative ions thus produced to migrate to the vane 21 and produce a flow of negative ion current. The negative ion current ilowing from the volume of air lying between the plates 16 andl be considered as substantially due to the radioactive material 17 lining the inner portion of that chamber. That is, due to the relative sizes of the air volume of the chamber 11 and the air space lying between the'plates 16, any small ionization due to erternal radiation can be considered as exclusively due to the chamber 1.1. Thus, to review briefly, there is present a negative ion current which may be considered a reference ion current due exclusively to a radioactive material 17 lining the inner face of an ion chamber constituted of the plates 16 andthe vane element 21. A positivelion currentllows from the chamber 11 and the electrometer plates 15- which can be-considered as being exclusively dueftottheexternal radiation which it is desired tomeasure.

The `current versus voltage relationship whichexists in` standard ionization chambers` of theY type illustrated in Figure l, is shownzin Figure Zot. the drawings where it can be seen that as a rough approximation, the current inthe ion chamber at low voltages,isproportional to the squarelroot of the:radiation. intensity, while at higher excitation voltages, a saturation current proportional to the radiationlevelzilows. For different radiation levels, this saturation current has different magnitudes. Thus, the positive ion current which will ilow inthe system from the chamber 12 plus the volume represented by the plates 15 will vary with the radiation intensity as is illustrated quite clearly in Figure 2.

The negative ion current which is due almost exclusively Vto :the presence of the radioactivevfoil 17 within the .plates 16 may similarlybe represented by a curve of the type` showninFigure 2. That is,fone` portion of the curveV illustrating therange wherein a current is proportional to thesquare root of the radiation intensity, while the remaining portion represents aV saturation current condition. However, the specific characteristics of this curve is a function ofthe position of .the vane 21 within the 'plates 16. Thus, a series of characteristic curves for the negative ion current, ofV the type. illustrated in Figure.`2maybe drawn, the various curves representing different'positions of the vane 21 within the plates 16. Thus, since the two ion chambers may be represented by a seriesA of characteristic curves respectively, it is possible to determine the distribution of the voltage. across the chambers, 'and consequently the magnitude of`current llow by `a graphical solution of the type illustrated in Figure 3a. p

Upto thispoint the'plateslS, 1'6 andthe vaneelement 21 have beenfdiscussed'exclusively in terms of their action as ion 'chamber elements. However, as is pointed out in "the specification, these 'elements also act as an electrometer apparatus. vThat is, since the movable vane 21 may assume different angular positions with respect to the electrometer plate elements 15`and'16, the mechanical torques presentinvthe system andltheir relationship to the.voltages,.ion currents, and` radiationlevelsmust be examined. In-an-electrometer of the type illustrated in Figure v1 the mechanical torqueon the Yvane element 21 is independent of position. That is, for any angle 0' The mechanical torque on the vane 21 is a function of the voltage squared (that is, thepotential difference between the vane 21 and the stationary electrometer plates) and the rate of change of capacity with respect to angular position. Thus,

. da 2.. T gv d t2 Combining Equations 1 and 2 the torque may then be defined as l T=W2K 3) Consequently, it can be seen that the mechanical torque on the movable vane element 21 is a function of the voltage squared and does not depend on position.

It is also clear that the electrostatic forces, and consequently the mechanical torque, exerted on lthe vane element 21 by the electrometer plates 15 and 16 respectively are opposed. This is quite apparent when it isy considered that the electrometer plates are connected to the positive terminal of the battery 18 whereas the electrometer plates 16 are connected to ground and the negative terminal of the battery 18. As a consequence, potential differences (V1, V2). exist between the movable vane`21 and the electrometer plates 15 and 16 respec-k tively which tend to produce opposing electrostaticforces and hence opposing torques. Y

For the system to come to equilibrium and for the vane 21 to assume a stable position, the potential difference between the vane 21 and the respective rvelectrometer plates 15 and 16 must be equal. This is obvious from an examination of Equation 3 above whichpindicates that thel mechanical torque on the vane 21 is the function of the voltage squared and, consequently, for the torques to be equal the voltages relative to the respective electrometer plates must be equal. To recapitulate brieiiy, in order for the electrometer vane 21 to reach a position of mechanical equilibrium, it is necessary that the electrostatic forces and consequently the torquesacting thereon beequal; that is, A v

y n V1=V2 l n addition, in order that this system may reach equilibrium and provide an indication of the radiation levels being measured, it is necessary that the system reach electrical equilibrium. That becomes obvious when it is recognized that the vane element 21, the rod element 13, and the connecting 'link 19 are highly insulated from the rest of the system and, as a result, it follows that in any stable condition of the system the net current ilow to this system must be zero. Stating it another way, the positive ion current produced by the radiation impinging on the-system must equal the negative ion current pro-v duced by the radioactive foil element 17 positioned within the .electrometer plates 16. This may be achieved, broadly speaking, by controlling the position of yvane 21. relative to the electrometer plates 16, sinceas pointed out previously'itis possible to control the amount of negative ion current drawn from these electrometer plates until equality between the ion currents is achieved. That isv the characteristic curve for the negative ion current is manipulated by controlling the position of the vane 21 until equalityv is achieved between positive and negaf tive ion currents.

The manner in which this is accomplished can be most readily understood in conjunction with the subsequent graphs which Aare a moreedetailed illustration of the araphioal solution shown .in Figure 3 of the application..

assumed a xed'an'gular position A representing an equi-` librium condition both mechanically andV electrically. In Fig'urebthis condition is illustrated graphically. v This ligure illustrates, by means of the solid line, the charac-V teristic curve forV the negative ion lcurrent I-v for a ixed position vA of the vane element 21. For equilibrium to exist in the apparatus, a condition postulated for the `immediate purpose, a radiation level Rs must exist. The characteristic curve of the chamber 11 and plates 15 is illustrated by the dashed curve which, for one portion thereof, is superimposed on the characteristic curve I. As can be readily seen from this curve, the positive ion current as'represented byY the dashed line has a magnitude, in a saturated condition, which is equal to the negative ion current represented by the solid line. As a consequence, the net current flow in the system is zero. It is also clear that at this position of the vane 21 mechanical equilibrium must exist. That is, the torques exerted upon the vane 211 by the respective electrometer plate elements 15 and 16 must be equal. As a consequence,

for this condition.

Now, if the external radiation level changes to th value of R2, which is lower than that existing previously, the equilibriu`m condition of the device is upset. As can be seen from Figure 3c the characteristic curve for the positive ion'current, which is shown by the dashed curve, produces apositive ion current I+ which is of a lower magnitude than the negative ion current represented by the solid curve, which exists instantaneously with the vane 21 remaining iixed in its position A. Hence, an unstable condition 'exists since more negative than positive ion current ilows.

The result of this electrically unstable condition is that the distribution of the voltages V1 and V2 change in such a manner as to tend to bring the system into an equilibrium conditionv electrically. That is, the voltages change in such a direction as to bring the negative'ion current to a' value which is equal to lthat of the positive current. That condition being represented by the point a at the inter-l section of the two curves. It can be seenl that the voltage' V2; i. e., that existing between the Vane 21 and the electrometer plates 16, is reduced to l2 volts; and the voltage V1, that existing between the vane and the electrometer plates 15, is increased to 88 volts. A result of the change in voltage distribution between the vane and the respective electrometer plates is that the mechanical equilibrium of the system is upset. That is, since the torque applied to the electrostatic Vane 21 is a function'of the voltage relative to the respective electrometer plates, the described change in voltage distributioncauses a torque to be applied to the vane 21 which draws it in the direction of the electrometer plates 15 and away from the electrometer plates 16. This movement of the vane 21 continues until the voltage V1 is once more equal to the voltage V2, the only stable voltage distribution for the system.

As a result of the physical movement of the vane 21,

the vane assumes a new position of equilibrium relative to the electrometer plates 16 which may be denominated as position B. The effect on the characteristic curve of the negative ion current of this move is illustrated in Figure 3d.

Figure 3d illustrates that the movement of the vane21 out of the electrometer plates 16 changes the characteristic curve of the negative ion current to such an extent' that this curve, illustrated by the solid line, is once again partially superimposed upon the positive ion curve, illustrated by the dashed line. As can be seen from this curve, the movement of the vane 21 to the new equilibrium position B reduces the magnitude of the negative ion current so thaty it equals the positive ion current ilowing due to the external radiation R2. Thus, the change in thekr magnitude of the external radiationchanges the' voltage distribution across. the electrometer, ,element developing 7 a `large correcting voltagewwhich causes themovable vaneto move outlof the 4electrorneter plates 16 :until a mechanical. ,eguilibriumY is achieved, fwhi'ch,simultaneously` changes'the-cbaracteristiccurve of the negativeion current .ow so `as tobring the system into -electrical equilibrium by equalizing the positive and `negative ion currents.

The sfactzthat the movable vane 21 has movedr further in between the. electrometer plates'. 15 does not, have any significant eiect on theoperation .ofthe` system since, as pointed out previously,.thepositive.ion current which tlows is `almost exclusively due to the ion chamber 11 due to the -relativesizes' of .theI two chambersfand, as a consequence, the ,position'of `the vane 21,relativ.e. to plates 15 causes a .negligible lchange `in the positive ion current .andiminimizesthefetects. of the electrometer plates 15.

Thus; it can beseen thatrelativelysmall changes in the external radiation level fproduce extremely large changes in .the 'voltage` distribution iacross the `electrometer portion of .thedeviee .This large changein voltage distribution causes a movementof the movable vane 21 in such a direction as to stabilize the system by controlling the magnitude of the negative ion current vflowing until it` cquals.fthepositivezion current dueto ithe external radiation. Hence, the system produces arelatively. large correcting voltage developed across. the .vane inV a direction to move the vane.back.to=its:balancedzpositionl by varying.

the rcharacteristic curve:of.the negative ioncurrent until it matches that of the ion chamber.l producing the positive ion current, Thus, a highly sensitive'and self-correcting radiation detectingapparatus isachieved by meansofthe instanticonstruction.

It isobvious from; theabove. discussion that shouldthe external radiation increase to R4, the mechanism .would operate .in a very. similar fashion. by increasing Vzand decreasingVr. Thisiwould draw'vane 21 towardsiplates. 16 and increasethe negativedon. current L. until it` equals the positive .ion current I+. due. to the radiationlevel at which time` anew mechanical and electricalequilibrium conditionwouldexist.

The stability which the` instrument incorporatesk can better be appreciated from an examination of the graph illustrated inFig.` 4 of'the drawings wherein a possible torque per volt squared characteristic of the instrument is plotted against the detlection of the movable vane 21. Such a characteristic would makean electrostatic ratio meter'where .i the deflectiondepends on the ratio-of V1 to V2.. This is not necessarily a more desirable condition thanconstant. torque'efliciency but. is taken to illustrate conditions when .constant efficiency does not'exist. The torquesdue to V1 andVz would have to be equal at a point.of stability. As shownat 30 the torques are equal if V1=V2, at zero degrees a change in the voltage as shown in Fig. 3. Thischange in voltage can produce a large change in torques for a small change in position-theoreticallyI it^ would -be possibleto geta change of the whole battery voltage fora very minor displacement. This is-in sharp'contrast to a conventional design, and produces large torque gradients with respect to position. Now if the `condition of'Fig. 4 existed,the analysis as shown inFig. l3'would stillapply. Asshown-there foriRs, Vi can be-any voltage1between 4.2,0 volts `and. .volts and: Vr correspondingly-.bw tween.. ,80-volts.and:20 volts. In` an actual case thefhoriA zontlpartsof the lcurves in Fig. twilhhave aminorslope and thisA will slightly modifyxth'e analysis. the calibration would 4bedone withfaradioactivefsource and' not by graphical 4ctmstruction which is-givemhere only to better explain the action.

A second embodiment of-.amcasuring instrument constructed in` accordance with the invention is illustrated in Fig; 5 of the drawings. This embodiment of the i`n vcntion comprises ra -tirst radiation'sensitive' chamber 23 formed by a conventional-ionization chamber havin'gan` outer` cylindrical electrode' 24 andaninn'errod-like elet:-4 trode 25. A second radiation sensitive chamber `26'is connected in electrical series circuit relationship with the first radiation sensitive .,chaml5er23, and comprises an outer cylindrical electrode `27 and.au inner rodflikelectrode 28 connected' to the outer cylindrical elect'roffe(` of the first ionization chamber 23. The outer cylindricall electrode 27 of the second Yradiation sensitive chamber 26 has a coating29 of radioactive.materialformed over' the interior, surface thereof, andl1a's. a rotatable shieldl mounted therein.for controlling Athe amount of radioac; tivefmaterialtexposed tothezinterior of thechamber. A source-ot energizingpotential comprisinga battery 30 is connected across. the serics.connected ionizationchambers, and anindicatinginstrument. comprising a conventional.quadrnatelectromcter having the twosets of di-` agonallyt opposed fixed' electrometer plates 32 yand 33, andi a movabletvane f3.4,..isconnected across the series connected' radiation sensitive chambers-'23E and y26. The movable` vanei34 of.` the` quadrantA electrometcr is connected` directly toitheinner rod-like electrodev 281andto theouter cylindrical electrode` 24, one--set of.diago'nally opposedixed plates 32 are connected tothefinnerrod-likeelectrode 2'5 offtheftirstE radiationfsensitivechamber'Q/B, and the remaining'diagonallytopposite vset` of 'fixed relectrom eter-"plates1533I are connectedI to the Aouter cylindrical electrode 27 off thesecondrr-adiationsensitivel chamber.. In this fmannerf two diagonally opposite setsof'xed electrometerv plates are connected effectivelyl across theenergizingsource L32, and the movable vane 34 is'connected to an equivalentmid-tap point on the series connected radiation' sensitive chambers.

In'operati'on, the embodimentv of the invention shown in Fig. 5ofthe drawings is entirelyA similar to that shown in Fig. lfanddescribed in connection with relation to Figs. 2 through 4. In particular, a plot of the ion currents owing in the series connected radiation sensitive chambers 24and 27 isfidentieal. to that shown in Fig. 3 of the drawings.v Hence, `very substantial changes in the energizingyoltage applied to thetseries connected radiation sensitive chambers, does not affect the ionization currentflowing therethrough. Likewise, should a change in,radiation level-occur; avery large change in voltage across the .opposite ionization chamber results so that large torques ,are available Afor moving` the indicating va'ne34.l 4Toritacilitate operation vof ythe instrument, the adjustable shield 31 is provided so that the ionization current owingzinftheradiationfsensitive'chamber 26 can be adjustedto equal thenormal background radiations reachingtheradiation sensitivechamber 23. Subsequent to. suchadjustment, shouldthe intensity of the. radiations reaching the` chamber `23'.c:hange,;this.change results in` the very. large/changesinvoltage appearing across the chamber-12.6,.and..willA be' indicated by the electrometer 34; Fromitliisl description,- it can be appreciated that many-Sofi` the .-a'dvantag'esof the structure shown in Fig.` l ofltheldrsawing; areV incorporated intthe embodiment of the"inventionlshown-in Fig. 5. Here the' null balance is obtained manually and theelectrometer is used only to\in'dicatea roughfbalan-ceof voltages. The electrometer .requirements are-notsevere; it need not be very sensitive'because thelvoltage-changes are Ilarge and it neetln not-.evenl bel particularly stable for the same reason.:y

-post 47 mounted thereon.

iStill a third form of the invention is illustratedvin Fig. 6 of the drawings which differs from that illustrated in Fig. l only in the construction of the indicating electrometer and second radiation sensitive chamber, otherwise the` instruments are entirely similar. Consequently, like parts in the two instruments will be labeled by the same reference numeral. The instrument shown in Fig. 6 comprises a first radiationsensitive chamber 11 comprised of an outer cylindrical electrode 12 .and an inner rodlike electrode 1-3. A -second radiation sensitive chamber is formedwby the two sets of spaced apart, fixed electrometer plates 36 and 37 of a .binant electrometer. The set Iof fixed plates 36 -is connected to the outer cylindrical electrode 12 of the first radiation sensitive cham-` ber .and to one side of a source of energizing potential 38, while the set of fixed plates 37 are connected throughl -asuitable conductor to the remaining side of -the source of energizing potential 38.v The inner surfaces of `each of the two fixed electrometer plates 37 have a radioactive material coated thereover, and a movable vane 39 is disposed between the spaced apart plates in each of the sets 36 and 37. The movable vane 39 is electrically connected to the inner rod-like electrode 13 of the first radiation sensitive chamber 11.

In operation, ion current through the first radiation sensitive chamber 11 is conducted to the movable vane 39 where it is neutralized by an ion current supplied thereto from the radioactive coating on the fixed electrometer plates 37. In the manner similar to that described with relation to the embodiment of the invention shown in Fig. l, when the two ionization currents are equal, the movable electrometer vane 39 assume an equilibrium position. Should the level of radiation reach in the radiation sensitive chamber 11 thereafter change, a large change vin voltage occurs between the vane 39 and the platesf37, and results in rotating kthe vane 39 so as to cause the vane to seek out or assume new equilibrium position andvthereby indicate a change in the radiation level. The voltage versus current characteristics of the instrument shown in Fig. 6 is identical to that shown in Fig. 3 of the drawings, and hence the instrument incorporates all of the advantages of the embodiment shown in Fig. 1 in that it is substantially independent of changes inthe energizing voltage 38. Likewise with regard to mechanical shock and vibrations, the instrument shown in Fig. 6 is extremely stable since there is a relatively large change in voltage should the vane 39 be mechanically moved from its equilibrium position, which change in voltage develops a comparatively large restoring torque for returning the vane to its equilibrium position.

A practical design for an instrument constructed in accordance with the principles of the present invention is illustrated in Figs. 7 and 8 of the drawings. Operation of the instrument shown in Figs. 7 and 8 is identical to that shown in Fig. 6 of the drawings in that it utilizes a radioactive material-coated binant electrometer in combination with an ionization chamber. The instrument comprises a circular base member 41 having an open ended cylindrical body member 42 mounted thereon. The body member 42 has an electrically conductive inner wall surface which coacts with an inner rod-like electrode 43 to form an ionization chamber. Secured to the remaining open end of the body member 42 is a supporting platform 44 of insulating material which has a pair of supporting arms 45 and 46 and a supporting Secured to the supporting post 47 are two sets of spaced apart fixed electrometer plates 48 and 49, best seen in Fig. 8 of the drawings, with the fixed electrometer plates 48 having a coating of radioactive material on the inner surfaces thereof. Secured to the end of the supporting arms 45 and 46 are a pair of bearings in which a rotatable shaft 51 is journaled. Secured to the rotatable shaft 51 is a movable vane 52 which is disposed into space between the spaced apart fixed electrometer plates 48 and 49, and is rotatable therein. The vane 52 is electrically conductive, and is connected through the rotatable shaft 51 to the inner rod-like electrically conductive electrode 43. The rodlike inner electrode 43.is supported by the jewel bearing on the end of supporting arm 46 in the chamber formed by the cylindrical body member 42. A suitable closure formed by an open-ended cylindrical member 54 and a circular cap 55 may be provided for enclosing the fixed f electrometer plates and the movable vane 52 within a protective housing.

The operation of the arrangement shown in Figs. 7 and 8 of the drawingr is exactly like that illustrated schematically in Fig. 6 of the drawing, and hence, will not be repeated. The arrangement has been described, however, for the purpose of disclosing a practical design of an instrument incorporating the features of the invention.

From the foregoing description, it can be appreciated that the present invention provides a new and improved highly sensitive radiation measuring instrument which is substantially independent of changes of energizing voltage supplied thereto, and which is dependable in operation. The instrument is not substantially affected by shock or mechanical vibration to which it might be subjected. ItV

is capable of developing an extremely large change in voltages for producing corrective torques required to overcome the friction of the indicating element whenever such element might be jarred or otherwise moved, and for the form in Fig. 5 is extremely sensitive in that it develops large changes in voltages for relatively smalll changes in radiation intensity. Additionally, the instrument is capable of being provided ina compact relatively inexpensive form to manufacture.

It is believed obvious that other modifications and" variations of the present invention are possible in theA light of the above teachings. It is, therefore, to be understood that changes may be made herein which are within the full intended scope of the invention asV defined by the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A radiation measuring instrument including in combination rst and second radiation sensitive chambers connected in series electrical circuit relationship, said second chamber having a predetermined amount of radioactive material disposed therein, and movable means adjacent said radioactive material and operatively coupled to said first and second radiation sensitive chambers and responsive to current flow therein for indicating the presence of penetrative radiations.

2. A radiation measuring instrument including in combination first and second radiation sensitive chambers connected in series electrical circuit relationship, said second chamber having a predetermined amount of radioactive material disposed therein, movable means adjacent said radioactive material and operatively coupled to said first and second radiation sensitive chambers and responsive to current flow therein for indicating the presence of penetrative radiations, and a source of energizing potential connected across said series connected first and second radiation sensitive chambers.

3. A radiation measuring instrument including in combination first and second radiation sensitive chambers connected in series electrical circuit relationship, said second chamber having a predetermined amount of radioactive material disposed therein, means for controlling the amount of radioactive material exposed to said second radiation sensitive chamber, and movable means adjacent said radioactive material and operatively coupled to said first and second radiation sensitive chambers and responsive to current flow therein for indicating the presence of penetrative radiations. v

4. A radiation measuring instrument including in combination an ion chamber, an electrometer having at least two sets of spaced apart fixed plates and a movable vane disposed therebetween, a radioactive coating on the inner asians,

Surfases afar least Qnaqf. Sait! Setssfsnasstlapart elcctromster plates, Latl isniaatisn, ,chamber having the outer electrode fhereqf 'cqanestsctto th @maintes set' Qf spasm apart txedelsqtrodes and having niatlsr-1sfi trede theref @amuseert t@ the tnQvabls. vail Qf- Said,` slee trsmetenand a SQurCs-Qfjeaaraiz.,

amiss, the litteram @tmf/spacetime@ A radiation measuring instrument ssnnttisitl.a,atl.` electromstsrhavitis at lead Setsf Smead @Palit fixed plates, a movable vane disposed in the -spaeb'etw Said ,spaced apart plates, andadanted t0. attive therebetween, and a. radioactive, maferialtisiiossdsn theinasr surfaces Qf at least one @i Salidas@ of'saastl apart this@ @barometer platea..

6 Mediation measuraias-tiastrwttntsanding ir'ltcQm-fbination rfirst; and assenti ibriizatian, members C Qmtsted imanes elcctricalfcirsuit rslat nshipasaid Scendi? LaatiQnhamher,havinaradtiative material dispesd t erein, and@ minable Sbieldssppnrtedtnr saidsasst-iaaaafiqnlhamber for centralina the ammini:,sfradiaastive materialA exitosewthia the Chamber- 7- A radiationmeastlrine instrument including, itl, 20@-A hinatifm `first and ssond fianiaafisn Qhambstslsannestsd inw Series electrical irutL relationship,` said, seaotidfienza: tion Chamber having radinactivs material dimmed-therei'n, a movable shieldsupportedin said seon'lipnizatipn chamber for controlling the amount ofradigactiye material exposed within the chamber, ya source oiener'gizing potential connected acrossl said series connected ioniza`- tion chambers, and an indicating instrument connected across said series connected ionization chambers in parallel circuit relationshipwith said source of energizing potential.

8. A radiation measuring instrument comprising a quadrant electrometer having four sets of spaced apart fixed l plates arranged circumferentially around a central axis, two diagonally opposite sets of said fixed plateshaving a radioactive coating on the interior surface thereof, and a movable vane disposed in the space between said spaced apart fixed plates and movable therebetween.

9. A radiation measuring instrument comprising a quadrant electrometer having four sets of spaced apart Vfixed plates arranged circumferentially around a central axis, two diagonally opposite sets of said iixed pilates having a radioactive coating on the interior surface thereof, a movable vane disposed in the space between said spaced 12 aaatttxsd plates and movable therebetvvssn., attelettisal conductor 4interconnecting said pair of radioactive coated setsllofixed plates, an electrical conductor interconnecting the remaining diagonally disposed sets of fixed plates, a source olf energizing potential connected between said electrical conductors, and a source of electrical signals to be'm'easuredconnected to said movable vane.

@1*0; A radiation measuring instrument comprising a qpalrant electrometer having four sets of spaced apartl fixed vplates arranged circumferentially Varound a central axis, two diagonally opposite sets of said fixed plates hav ing radioactive coating on the interior surface thereof, a rnovable vane disposed in the space between said spaced apart fixed plates and movable therebetween, an electrical c dnductor interconnecting said pair of radioactive coated sets of fixed plates, an electrical conductor interconnectf ing the remaining diagonally disposed sets of iixed plates, a source of energizing potential connected'between said' electrical conductors, and an ionization chamber having the outer electrode thereof connected to one side of said source of energizing potential and having the inner electrode thereof connected to saidl movable vane.

1,1. A radiation measuring instrument comprising a circular base member, an open-ended cylindrical body member having a conductive inner surface,vmounted on said` circular base member, a supportingplatfom secured tothe end of said cylindrical body member opposite saidV basentember, a conductive rodsubtended from said sup-y porting platform and extending into said cylindrical body member, at least two sets of spaced apart fixed electror'nl eter plates mounted on said supporting platform exterior'. of said cylindrical body member, one of said sets of plates having a radioactive coating on the inner surface thereof, a movable vane disposed in the space between said spaced apart plates and movably mounted on journals supported on said supporting platform, said movable vane and `said conductive rod being electrically interconnected, and a cap member enclosing said fixed electron-eter plates and movable vane.

References Cited in the tile of this patent UNITED STATES PATENTS 2,610,302 Christian Sept. 9, 1952 2,668,245 Rich Feb. 2, 1954 2,676,270 Lahti Apr. 20, 1954` 

